我可以在 Cloud Firestore 的子集合中查询和检索文档的字段值吗？

Question

Can I query and retrieve the Documents' field values in a subcollection?

Take the following for example:

States [Collection]
    State 1 [Document]
        Counties [Collection]
           County 1 [Document]
               British Population = 100 [Field]

           County 2 [Document]
               British Population = 200 [Field]

    State 2 [Document]
        Counties [Collection]

           County 1 [Document]
               British Population = 150 [Field]

How do I query the Documents with British Populations and retrieve the number of the population ?

Answer 1

Since the name of the Counties subcollection is the same within all State documents, to get the value of the "British Population" property, a Firestore collection group query is required. However, this will not work unless you have another property under your Country document that can help you create that query. In code, should look like this:

db.collectionGroup("Counties").whereEqualTo("nationality", "British").get()
        .addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
            @Override
            public void onSuccess(QuerySnapshot queryDocumentSnapshots) {
                // ...
            }
        });

This is translated like this. Hey Firestore, give me all Country documents that exist in all Countries collections or subcollections where "nationality" is "British". So as you can see, I have added a new property named nationality and added the corresponding value as "British". Since all documents do contains such property, all documents will be returned.

Besides that, the "British Population" property can also be changed to "population". Now, inside the onSuccess() , you can query the QuerySnapshot object and get the value for the population property.